WO> U+F3A2 PLEASE LIGATE THE NEXT TWO CHARACTERS
WO> U+F3A3 PLEASE LIGATE THE NEXT THREE CHARACTERS
WO> U+F3A4 PLEASE LIGATE THE NEXT FOUR CHARACTERS

While I don't think this discussion of various PUA allocations should
continue very further, it's probably a lot better to introduce the
already-discussed ZERO WIDTH LIGATOR in such a form that X ZWL Y
produces the XY ligature, X ZWL Y ZWL Z the XYZ ligature and so on. It
saves you a lot of hassle with longer ligatures.

Does this belong in a character-based encoding system at all? This is
better solved by markup. If you go on defining your own file formats
already, do include some sensible markup system there, and you don't
have to clutter the PUA and restrict their use. What if you've got
more than 2 swash forms, BTW?